Method of controlling an aluminum reduction cell with the minimum power

ABSTRACT

This invention refers to metallurgy of aluminum, specifically, to the method of extracting aluminum by molten salt reduction, namely, the method of controlling an aluminum reduction cell by the minimum power. This method consists in measuring the resistive voltage drop in the reduction cell, comparison of the measured value with the set voltage drop value in the reduction cell and elimination of the mismatch by the relevant anode displacement. The anode displacement reduces the mismatch between the heating power and the set value until the minimum power is released in the reduction cell. Release of the minimum power is determined by the spontaneous growth of the electrochemical component of the reduction cell voltage, this mismatch is maintained with the relevant anode displacement without any variations in the thermal state of the reduction cell. The invention enables to reduce the electric power consumption, enhance the current metal yield and reduce the labor intensity of reduction cell maintenance.

CROSS-REFERENCE

The present application claims convention priority to Russian Utility Patent Application No. 2015110596, filed on Mar. 25, 2015, entitled “

”. This application is incorporated by reference herein in its entirety. The present application is a National Phase Entry of International Patent Application no. PCT/RU2015/000324, filed on May 22, 2015, entitled “METHOD OF CONTROLLING AN ALUMINUM REDUCTION CELL WITH THE MINIMUM POWER”. This application is incorporated by reference herein in its entirety.

Field of the Invention

This invention refers to aluminum metallurgy, specifically, the method of producing aluminum by molten salts reduction.

Prior Art

Today, aluminum smelters are equipped with microprocessor controllers (industrial controllers) for reduction cells. These controllers enable to achieve the maximum technical and economic indicators of the aluminum reduction process by improving the optimal control over the anode-to-cathode distance in the reduction cell. Using reduction cell microprocessor controllers ensures continuous measurement of potline current and resistive voltage drop in the reduction cell. This makes it possible to extend the potential of controlling the aluminum reduction process due to the prompt registration and compensation of disturbances introduced in the industrial process in case of potline current oscillations, changes in the counter EMF along with other variable parameters of the aluminum reduction process.

The maximum registration of disturbances introduced in the process, exclusion of their adverse impact on the stability of maintenance of the reduction cell anode-to-cathode distance (ACD) allow achievement of the maximum technical and economic indicators of the reduction process. This requires maintenance of the optimal ACD value in accordance with the changes in the thermal and process state of the reduction cell in each period of the reduction cell voltage control and maintenance of the optimal conditions of the process during the maximum period of time.

The following indirect electrical method for measuring the reduction cell pseudoresistance is used under industrial conditions:

R _(acd)=(U _(rc) −E _(O))/I _(c).

Such variable parameters as reduction cell voltage (U_(rc)), counter EMF (E_(O)), potline current (I_(c)) going through the reduction cell are continuously changing and require stabilization of the reduction cell resistance through adjusting the anode-to-cathode distance by anode displacement.

The reduction cell voltage is the sum of the voltage drops in its current-conducting parts:

U _(rc) =U _(busbar) +U _(cathode) +U _(anode) +U _(acd) +E _(O),

where U_(busbar) is the voltage drop in the reduction cell busbar, U_(cathode) is the voltage drop in the cathode, U_(anode) is the voltage drop in the anode, U_(acd) is the voltage drop in the ACD, E_(O) is counter EMF. This introduces an error in the determination of U_(acd,) as well as in maintenance of the stable thermal and process state of the reduction cell.

In accordance with the maintenance of the reduction cell heat balance,

Q _(rc) +Q _(anode) =Q _(decomposition) +Q _(metal) +Q _(gas) +Q _(losses),

where: Q_(rc) is the heat supply resulting from the electric energy; Q_(anode) is the heat supply resulting from the anode combustion; Q_(decomposition) is the heat consumption for the alumina decomposition; Q_(metal) the heat loss with the discharged aluminum; Q_(gas) is the heat loss with exhaust gases; Q_(losses) are external heat losses by the reduction cell structural elements; the stability of the thermal and process state of the reduction cell is determined by the heating power produced by the current flow:

Q _(rc)=0.86×I×U _(heat),

where 0.86 is the thermal equivalent, I is the electric current, A, U_(heat) is the heating voltage of the reduction cell, V.

At present, the optimal thermal and process states of the reduction cell are maintained by the reduced voltage as follows:

U _(red)=(U _(oper) −Eo)I _(red) /I _(potline) +E _(O),

where U_(oper) is the operational voltage of the reduction cell, Eo is the counter EMF, I_(rated) is the rated current, I_(potline) the potline current. These calculations are provided for stabilization of U_(acd) from variations in the counter EMF (E_(O)) and the potline current (I_(potline)) with the regular adjustment of the reduction cell, when E_(O) has the most probable mean value of 1.55 V.

Under conditions of continuous voltage measurement in the reduction cell with microprocessor controllers (U_(red)), indirect estimation is provided for the heating power and heat release, in accordance with the thermal balance of the reduction cell. This does not include any power changes in case of potline current changes, counter EMF, and other variable parameters with conventional constant values, as well as excluding the dead band (60 mV), where no reduction cell is adjusted; the required power is maintained randomly.

During the adjustment of the reduction cell, this adds an error in the assessment of the release of the required power in the reduction cell for the maintenance of the optimum process state of the reduction cell and achievement of the maximum performance indicators.

The maximum metal production and the minimum consumption of electric power are achieved with the minimum voltage in the reduction cell and the minimum alumina concentration in the bath that does not cause any spontaneous growth of the electrochemical component of the reduction cell voltage (counter EMF). Under the given process conditions, it is necessary to maintain the release of the minimum power in the reduction cell, register all disturbances included in the process and achieve the maximum ACD stabilization during the anode displacement. In this case, it is required to maintain the release of the minimum power in the reduction cell in accordance with the changes in the process state of the reduction cell, reduce losses related to cathode aluminum oxidation during the anode displacement, and exclude any unreasonable growth of the electric power consumption with respect to release of the minimum power in the reduction cell.

A known method of aluminum reduction cell control (RF patent No. 2166011, cl. C25C 3/20, published on Apr. 27, 2001) is based on concurrent measurement of the reduction cell resistance between the anode and cathode busbars of the cellular current lead on one of the end sections of the reduction cell and measurement of the resistance differential of the cathode busbars of the middle and the opposite end of the reduction cell. Anode is displaced on the basis of the reduction cell resistance, by deducting the resistance differential between the cathode busbars from it, taking into consideration the resistance differential sign. The maximum resistance differential between the cathode busbars is deducted from the resistance of the reduction cell; the anode displacement is performed on the basis of the reduction cell resistance at a section with the minimum ACD value.

The disadvantage of the known ACD adjustment methods (despite all their advantages) is the anode displacement by the reduced voltage with respect to the set voltage of the reduction cell and the dead band, as well as the fixed electrochemical component of the reduction cell voltage and the absence of automatic prompt adjustment of the reduction cell resistance under conditions of changing variable parameters of the reduction process and the process state of the reduction cell.

The closest equivalent (both technically and in terms of the result achieved) to the suggested method is the method of aluminum reduction cell control (RF patent No. 2202004, C25C 3/20, published on Apr. 10, 2003). Pursuant to the prototype method, the voltage drop is measured for the resistance of the reduction cell consisting of the electrochemical and ohmic components. The measured value is compared with the set voltage drop value in the reduction cell. The mismatch is eliminated by the relevant anode displacement. A voltage drop zone equal to the value of possible changes in the electrochemical component of the voltage drop is established with regard to the set voltage drop in the reduction cell. Anode displacement in this area is based on the mismatch between the heating power released in the reduction cell and the set value. The period of changing the heating power is taken equal to the period of eliminating the mismatch without changing the thermal state of the reduction cell.

The disadvantage of the closest equivalent and of the known methods for adjustment of reduction cells is the elimination of the mismatch between the heating power and the set value. The set value of the heating power is determined by the preset value for adjustment of the reduction cell, which is regularly selected by the potroom staff (once per day) by the frequency of anode effects and any other indirect process parameters characterizing the thermal states of the reduction cell. The set value of the reduction cell voltage indirectly determines the release of the required power to maintain the optimal process state of the reduction cell, with concurrent effect on the process by the counter EMF variations and any other variable parameters of the reduction cell resistance. This does not allow considering all variable parameters of the reduction cell operation, maintaining the minimum power release in the reduction cell during the maximum period, and achieving the maximum technical and economic indicators of the reduction cell operation.

BRIEF SUMMARY OF THE INVENTION

The objective of this invention relating to the suggested method is to enhance the technical and economic performance indicators of the reduction cells operation.

The technical result of the invention is the reduction of electric power consumption, increase in the current metal yield, reduction of the labor intensity of servicing reduction cells.

The technical result is achieved by the following: pursuant to the method of aluminum reduction cell control according to which the resistive voltage drop in the reduction cell is measured, then the measured value is compared with the set voltage drop value in the reduction cell and the mismatch is eliminated by a anode displacement, characterized in that the anode displacement reduces the mismatch between the heating power and the set value until the release of the minimum power in the reduction cell, such release of the minimum power is determined by the spontaneous growth of the electrochemical component of the reduction cell voltage, and this mismatch is maintained by the relevant anode displacement without changing the thermal state of the reduction cell. The mismatch between the heating power and the set value with the release of the minimum power in the reduction cell is created during the period of the reduction cell thermal time constant, and this mismatch is maintained in each period of measuring the resistive voltage drop in the reduction cell. The anode displacement is determined on the basis of the mismatch between the minimum power in the reduction cell and the set value during the current period of its operation and the predicted release of the minimum power during the further period of its operation. The release of the minimum power in the reduction cell without changing the thermal state of the reduction cell is maintained by the anode displacement by the value of possible power reduction in the reduction cell to the minimum value and the power increase by the value of the spontaneous growth of the electrochemical component of the reduction cell voltage. The release of the minimum power is determined by the spontaneous growth of the electrochemical component of the reduction cell voltage concurrently with all variable parameters of the aluminum reduction process from all disturbances introduced in the process.

The given power during the further period of the reduction cell operation is determined on the basis of the product of the given power during the current period of its operation and the mismatch between the minimum power and the set value during the relevant period of the reduction cell operation. Creation of the anode displacement mismatch between the heating power and the set value with the release of the minimum power in the reduction cell determines the required minimum release of the heating power for stable operation of the reduction cell, the optimal voltage of the reduction cell exclusive of variable parameters in order to maintain the optimal ACD, the maximum period of the reduction cell operation and ensures achievement of the maximum indicators of the reduction process.

Determination of the minimum power release by the spontaneous growth of the electrochemical component of the reduction cell voltage and maintenance of this mismatch with relevant anode displacements without changing the thermal state of the reduction cell excludes the dead band and expands the possibilities of adjusting the heating power of the reduction cell within the entire range of the measured voltages to maintain the minimum power in the current process conditions of the reduction cell operation during the maximum period of time.

Introduction of mismatch between the heating power and the set value with the release of the minimum power in the reduction cell during the period of the reduction cell thermal constant value and maintenance of this mismatch in each period of measuring the resistive voltage drop in the reduction cell without changing the thermal state of the reduction cell excludes the assessment error of the minimum power and the process state of the reduction cell from using the average value of the counter EMF, the variable parameters with conventional constant values as well as any potline current oscillations introduced in the process and performance of process operations in the reduction cell.

Determination of the anode displacement by the divergence of the minimum power in the reduction cell from the set value during the current period of its operation and the predicted release of the minimum power during the further period of its operation determines the optimal anode displacement to maintain the optimal process state of the reduction cell during the further period of its operation.

Maintenance of the minimum power release in the reduction cell by the anode displacement without changing the thermal state of the reduction cell by the value of possible power reduction in the reduction cell to the minimum value and power growth by the value of the spontaneous growth of the electrochemical component of the reduction cell voltage determines the optimal area of the reduction process stability to maintain the optimal ACD under the current process conditions of the reduction cell operation.

The calculation of the minimum power release by the spontaneous growth of the electrochemical component of the reduction cell voltage concurrently with all variable parameters of the aluminum reduction process from disturbances introduced in the process provides an integrated analysis of all variable parameters of the reduction cell operation and their optimal compensation under the current process conditions of its operation.

The calculation of the set power during the further period of operation of the reduction cell by the sum of the set power value during the current period of its operation and the mismatch between the minimum power and the set value during the relevant period of operation of the reduction cell determines the optimal set voltage of the reduction cell during the further period of its operation by the objective electrical parameters of the mismatch between the minimum power and the set value.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1—The table showing the parameters of comparative efficiency of adjusting industrial reduction cells.

FIG. 2—The voltage diagram of industrial reduction cell No. 009, adjusted by industrial controller PCS “SHUEBM” by using the known process solution.

FIG. 3—The voltage diagram of industrial reduction cell No. 008, adjusted by industrial controller PCS “SHUEBM” by using the suggested method.

DRAWING SYMBOLS

FIGS. 2 and 3 show two diagrams of the reduced voltage U_(red) of similar industrial reduction cells No. 009 and No. 008 of potroom No. 5 of RUSAL Bratsk smelter operating under the same process conditions, within the same period; the reduced voltage U_(red) of the reduction cell is adjusted by the industrial reduction cell controllers PCS “SHUEBM” using the known and suggested methods.

The reduced voltage diagrams show that the left axis determines the voltage of the reduction cell, in Volts; the right axis indicates the intensity of the current flowing in the reduction cell, in kiloamperes. The lower axis characterizes the 24-hour period of the current day divided into 5-minute intervals of the control time and adjustment of the reduction cell voltage; it shows the examples of 288 values of the measured and adjusted parameters.

The following are marked with digital symbols in the diagrams:

1—the dead band ±30 mV with respect to the central line of the set voltage of the reduction cell (U_(set)),

2—time variations in the reduced voltage of the reduction cell in case of adjustment by the known and suggested methods,

3—arrowheads indicating the time and direction of the anode displacement when adjusting the reduced voltage of the reduction cell,

4—the line indicating the maintenance of the anode displacement of the minimum power in case of the reduction cell adjustment under the current process conditions of its operation,

5—time variations in the operating voltage (U_(oper)) measured in the reduction cell,

6—arrowheads indicating the time and direction of the anode displacement when adjusting the minimum power of the reduction cell.

The maximum divergence peaks of the reduced voltage in dependences 2 indicate the disturbance effect included in the process when an anode effect occurs;

Dead bands 3 are eliminated by adjusting the minimum power of the reduction cell, which allows maintaining the minimum power throughout the entire range of measured voltages and during each period of the reduction cell operation time in order to achieve the maximum metal production and the minimum electric power consumption.

Embodiment of Invention

The parameters of comparative efficiency of adjusting industrial reduction cells are listed in the table in FIG. 1, showing the advantage of the suggested control method over the known technical solutions.

When applying the control method by the known technical solutions (see FIG. 2), the formula for calculation of the reduced voltage

U _(red)=(U _(oper) −E)I _(red) /I _(potline) +E,

uses the average value of the counter EMF of 1.55 V, variable from 1.3 to 1.8 V, the dead band of 60 mV, and does not consider power changes whenever potline current changes, as well as variations in any other variable process parameters and variations in the operating dynamics of the reduction cell and its process state.

The known control method does not consider any difference in the heating power release under conditions of changing the voltage of the reduction cell with the range of 4,270-4,330 mV. No reduction cells are adjusted within the 60 mV range of the dead band; the maximum operating performance indicators are achieved randomly.

The operating conditions of the reduction cell corresponding to the minimum electric power consumption and the maximum metal production are randomly maintained in the reduction cell with regard to the dead band and selected by the potroom process staff by changing the preset values for adjustment once per day (not promptly).

The restricted capabilities of the known control method do not allow integrated registration of all disturbances introduced in the process, performance of prompt overall assessment of the process state of the reduction cell, which reduces the efficiency of use of industrial reduction cell controllers as well as prevents from achieving the maximum indicators of the aluminum production and reducing electric power consumption.

When applying the suggested method of aluminum reduction cell control (see FIG. 3), the minimum power curve is calculated on the basis of the analysis of several parameters and the dynamics of their changes; it characterizes variations in the thermal and process state of the reduction cell with respect to the set voltage.

The voltage of the reduction cell and the ACD value are maintained with respect to the minimum power curve considering any changes in the variable parameters of the reduction cell operation and its process state, which enables to improve the optimal adjustment of the reduction cell and achieve the maximum indicators of the reduction process within each five-minute period of its operation time.

The suggested control method eliminates the dead band; ACD is maintained throughout the entire range of measured voltages considering the current process state of the reduction cell.

In 288 values of the changed and adjusted parameters, the divergence of the adjusted minimum power parameter of the set value changed by 74 mV, which, in accordance with the change step of the given 10 mV voltage made it possible to maintain the minimum power within the range of 7 preset values of the optimal set voltage for each 5-minute period of operation of the reduction cell. Changes in the variable operation parameters of the reduction cell with the optimal maintenance of the counter EMF (Eo) along with any other variable parameters of its operation were automatically and promptly registered within the set voltage range.

The suggested control method makes it possible to control the metal production conditions and electric power consumption, select the adjustment conditions that ensure fewer unreasonable interelectrode gap changes; the reduction cell achieves the maximum performance and maintains it for the maximum period of time with uninterrupted power supply and periodic alumina supply in the reduction cell, as well as with introduction of any other disturbances in the process.

The table (FIG. 1) shows that the ACD maintenance stability is increased by 25-30% due to the reduction in the number of displacements and the distance of the anode displacement in case of adjustment of the reduction cell by the suggested method. At the same time, the reduction cell voltage is reduced by 30-40 mV. The maximum metal production and the minimum electric power consumption are achieved within each five-minute period of operation of the reduction cell, which is an additional technological reserve of enhancement of the performance indicators of the existing reduction cells when the suggested method of aluminum reduction cell control is applied.

The achieved industrial test results enable to reduce the electric power consumption by 250-300 kWt/hours per ton of produced metal, enhance the current metal yield by 0.5%, expand the amount of visualized information about the process stage of the reduction cells in the potroom, which allows the personnel to provide more objective maintenance of the process and reduce the labor intensity of reduction cell maintenance operations by 5-10%.

The suggested method of the aluminum reduction cell control by the minimum power may be used in all modifications of aluminum reduction cells manufactured by domestic and foreign aluminum smelters.

The industrial tests of this method were performed during a year, with a group of 6 reduction cells of RUSAL Bratsk smelter. 

1. The method of controlling the aluminum reduction cell by measuring the resistive voltage drop in the reduction cell, comparing the measured value with the set voltage drop in the reduction cell and eliminating the mismatch by displacing the relevant anode characterized by displacing the anode, thus reducing the mismatch between the heating power and the set value until the release of the minimum power in the reduction cell; determining the release of the minimum power by the spontaneous growth of the electrochemical component of the reduction cell voltage and maintaining this mismatch by the relevant anode displacement without changing the thermal state of the reduction cell.
 2. The method of claim 1 characterized by creating the mismatch between the heating power and the set value with the minimum power release in the reduction cell during the period of reduction cell thermal constant and maintaining in each period of measurement the resistive voltage drop in the reduction cell.
 3. The method of claim 1 characterized by determining the anode displacement by the mismatch between the minimum power in the reduction cell and the set value during the current period of its operation and the predicted minimum power release during the further period of its operation.
 4. The method of claim 1 characterized by maintaining the minimum power release in the reduction cell without any changes in the thermal state of the reduction cell through displacing the anode by a possible power reduction value in the reduction cell to the minimum value, and increasing the power by the value of spontaneous growth of the electrochemical component of the reduction cell voltage.
 5. The method of claim 1 characterized by determining the minimum power release by the spontaneous growth of the electrochemical component of the reduction cell voltage concurrently with all variable parameters of the aluminum reduction process from all disturbances introduced in the process.
 6. The method of claim 3 characterized by determining the set power for the further period of operation of the reduction cell by the sum of the given power value during the current period of its operation and the mismatch between the minimum power and the set value during the relevant period of the reduction cell operation. 